Metal hexaborides (MB6) are refractory ionic crystals with unique thermophysical properties including low work functions, chemical inertness, and high hardness values among other. Also, they have a broad variety of potential applications such as thermionic emitters, corrosion-resistant coatings, sensors, and catalysis among many other. Ternary mixtures of metal hexaborides offer additional possibilities to tune material properties for many of the applications aforementioned. In this work, we use density functional theory (DFT) calculations to stability of the solid solutions of mixed-cation hexaborides and to study how the spatial arrangement of cations within the mixture can affect properties related to stability. Bond-lengths within the boron framework are found to be heavily dependent upon the local cationic environment, and energies calculated at zero Kelvin suggest certain stoichiometries naturally lead to phase separation. Some of these calculations and conclusions are also complemented with recent experimental characterization of solid solutions of these materials using X-ray diffraction analysis.